The inventor's assignee has proposed in, e.g., Japanese Patent Application Nos. 2-221364 (1990), 2-221365 (1990), 2-222821 (1990), and 2-222823 (1990), a technology for compressing a digital audio input signal and recording the compressed digital signal on a recording medium in bursts of a predetermined volume of data.
With this technique, a magneto-optical disc is used as the recording medium on which is recorded adaptive differential PCM (ADPCM) audio data, as prescribed in the CD-I (CD-Interactive) or CD-ROM XA audio data formats. The ADPCM data is recorded in bursts on the magneto-optical disc, with, e.g., 32 sectors of the ADPCM data together with several linking sectors, which are used to accommodate interleaving, as a recording unit.
In a recording and reproducing apparatus employing a magneto-optical disc, one of several recording and reproduction modes may be selected for the ADPCM audio data. Modes A, B, and C, in which normal CD data is compressed with compression ratios of two, four, and eight times, have been defined. In mode B, for example, the digital audio data is compressed with a compression ratio of four, so that the maximum playback time of a disc recorded using mode B is four times that of a disc recorded with the standard CD format (CD-DA format). This enables the size of the apparatus to be reduced because a playing time comparable with that of a standard 12 cm CD can be achieved using a smaller-sized disc.
The recording track velocity relative to the pickup head (the "recording velocity") of the smaller-sized disc recorded with Mode B compressed digital data is chosen to be the same as that of a standard CD. This means that the volume of compressed digital data reproduced from the disc per unit time is four times the volume of digital data required by a Mode B decoder. For this reason, the same sector or cluster of compressed digital data is read from the disc four times, but compressed digital data corresponding to reading the data from the disc once is sent to the decoder.
The data is recorded on the disc on a spiral track. When reproducing the track, the head is caused to execute a radial track jump each complete revolution of the disc. The track jump returns the head to its original position on the track. Causing the head to execute four track jumps causes the head to read the same part of the track four times repeatedly. This method of reproducing the track is advantageous, especially when used in a small-sized portable apparatus, since it enables satisfactory reproduction to be obtained even if accurate compressed data is obtained in only one of the four readout operations. The method therefore provides a strong immunity against reproduction errors caused by physical disturbances and the like.
In future, semiconductor memories are expected to be used as a recording medium for digital audio signals. To enable semiconductor memories to provide a usable playing time, it is necessary to increase the compression ratio further by using variable bit rate compression encoding, such as entropy encoding. Specifically, it is anticipated that audio signals will be recorded and/or reproduced using IC cards employing semiconductor memories. Audio data compressed using variable bit rate compression will be recorded on and reproduced from the IC card.
Although the playing time of an IC card is expected to increase, and the cost of an IC card is expected to decrease, with progress in semiconductor technology, it is feared that the IC card, which has barely started to be supplied to the market, will initially be expensive and have a short playing time. Therefore, it is thought that an IC card might early on be used by transferring to it part of the contents of another, less expensive, larger capacity, recording medium, such as a magneto-optical disc. Data exchange and re-recording operations would be conducted between the IC card and the magneto-optical disc. Specifically, a desired one or more selections recorded on the magneto-optical disc may be copied to the IC card. The copied selections would then be replaced by other selection(s) when desired. By repeatedly exchanging the selections recorded on the IC card, a variety of selections may be played on a portable IC card player using a small number of available IC cards.